Vehicles have been designed to capture and store fuel vapors in carbon canisters to comply with emissions standards in a variety of markets. In some vehicles, such as vehicle's designed with stop-start capabilities, the engines may have limited run times and therefore may overload the carbon canister. For instance, during an idle-stop condition fuel stored in a fuel tank will continue to vaporize and load the canister. Overloaded canisters present a variety of problems, such as an inability to purge the canister by a desired amount due to scheduled drive cycle diagnostic routines that cannot be implemented in tandem with canister purge operation.
Attempts have been made to remedy this problem by installing a vapor blocking valve between the canister and the fuel tank. The vapor blocking valve may be closed to completely seal the fuel tank during conditions such as canister purge operation, a key-on condition, etc., and opened during other conditions. In this way, during idle-stop canister loading is prevented. However, completely sealing the fuel tank with the vapor blocking valve causes fuel tank pressure buildup. The pressure buildup in the fuel tank may necessitate a purge strategy that slowly ramps up vapor purge to avoid engine stalls caused by a fuel vapor spike (e.g., vapor slug) in the intake system. However, slowly ramping up vapor purge creates a purge efficiency penalty and therefore leaves a smaller window open to purge the canister during a drive cycle. As such, vapor blocking valves have been designed with notches to reduce the amount of fuel vapor buildup in the fuel tank. Consequently, more efficient vapor purging may be carried out while reducing canister loading during idle-stop.
However, previous diagnostic routines where a vacuum is generated in the fuel tank and threshold pressures are used to determine if a leak is occurring in the vapor recovery system are not applicable to systems employing notched vapor blocking valves due to the gas flow through the notch. For instance, U.S. Pat. No. 9,243,591 discloses a diagnostic technique for a vapor recovery system. In the diagnostic routine, a vacuum is generated in the fuel tank and during a subsequent a bleed-up phase the rate of bleed-up is compared against a threshold. However, this diagnostic technique is not compatible with a system having a notched vapor blocking valve because the notch will adversely affect the bleed-up rate. Furthermore, the bleed-up threshold disclosed in U.S. Pat. No. 9,243,591 is limited to the specific design of the vapor recovery system. As such, the threshold bleed-up rate may be separately calibrated for different engine designs, driving up costs and creating barriers that may limit the system's applicability.
To address at least some of the aforementioned problems a method for diagnosing an evaporative emission control system is provided. The method includes during a first state of a vapor blocking valve, determining a first rate of change of a vacuum in a fuel tank, during a second state of the vapor blocking valve different from the first state, determining a second rate of change of the fuel tank vacuum, and diagnosing an operational condition of the vapor blocking valve based on the first and second rates of change. When multiple rates of change of the fuel tank vacuum are used for diagnostics a more robust and reliable diagnostic routine can be achieved. In one example, the first and second rates may be compared to determine the operational state of the vapor blocking valve. When the diagnostic routine utilizes a vacuum bleed-up rate comparison the diagnostic routine may be applied to a variety of vapor recovery systems with differently sized notches, fuel tanks, vapor storage canisters, etc., without having to recalibrate diagnostic thresholds, if desired. Consequently, the applicability of the diagnostic technique is broadened.
In one example, the vapor blocking valve allows a metered fuel vapor flow there through in a closed state. In this way, the fuel tank pressure buildup during idle-stop conditions can be reduced while reducing the amount of vapor canister loading during such conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.